1. Field of the Invention
The present invention relates to a method of calendering a paper web or cardboard web by applying pressure, moisture and heat in a press nip.
2. Description of Background and Relevant Information
During the calendering of paper or cardboard it is known that the structure achieved in the press nip of the calender alters after departure of the paper web from the press nip. There thus occurs a change in the paper web, whereby the desired smoothness of the surface which has already been attained in the press nip again decreases, that is to say, the roughness of the surface increases. This occurs because of a more or less pronounced and localized varying re-swelling, and specifically, especially in the presence of calender temperatures in the press nip which are beneath the glass transition point (i.e., the plastification temperature) of the material to be calendered.
A reduction in the smoothness is usually also then observed when the calendering temperatures in the press nip are above the glass transition point and the paper web, following departure from the press nip, gradually cools at the ambient temperature below the glass transition temperature of the material. One speaks of the paper "working".
Frequently, the optimum surface characteristics of the calendered paper or cardboard desired for the further processing, such as for imprinting or inscribing and so forth, as described in German Patent Publication No. 3,600,033, only can be obtained with high temperatures of the surfaces of the employed rolls of the calender or only with high temperatures in the press nip, respectively.
In many cases it is recommended to heat the paper or cardboard web at least at its surface layers above the glass transition temperature, in order to here attain the plastification of the material. After departure from the press nip the warm web gradually again thermodynamically comes into equilibrium with the surroundings, that is, the differences in the temperature and moisture content are compensated in equilibrium with the surroundings. In so doing, the material "works", and that much more intensively the greater the starting temperature of the web. This is a known long-time effect, wherein the micro-roughness of the smoothed surface can again increase. The mentioned elastic re-swelling additionally occurs, and the non-plasticized inner layers of the web increase in thickness in relation to the condition in the press nip. This is a known short-time effect which is completed after about 100 ms to 2 minutes following departure of the web from the press nip. This re-swelling is locally different. It is particularly pronounced at locations of greater compression, such as, for example, locations containing fiber flocks or locations possessing greater surface weight in the web. Consequently, there likewise arises an increase in roughness at the fiber flock region after calendering, i.e., "macro-roughness".
All of this can have the consequence that for achieving a desired high surface smoothness of the paper web the calendering operation must be carried out a number of times in succession, for example, in a number of press nips with varying calender conditions as concerns the pressure, the moisture content of the paper web and the temperature, or the operation must occur more slowly or with greater line force.
According to past experience the following enumerated operating parameters enhance the operation in which the paper web located in the press nip of the calender at a temperature above the glass transition point of the material should be brought below the glass transition point of the material following the departure from the press nip of the calender:
(a) If the paper web to be calendered is thick, that is, has a high surface weight, then the thermal energy taken up in the press nip is removed in the interior of the web, and there occurs a rapid cooling of the surface provided that higher temperatures prevail in the surface layers than in the interior of the web and the mean temperature is below the glass transition temperature;
(b) If there is present a corresponding high web moisture content, then the paper web, following departure from the press nip, is cooled by the cold due to vaporization, especially if the paper web is heated in the press nip above 100.degree. C;
(c) If the degree of crystalinity of the material is high, and thus, also the glass transition temperature in the paper web, so that then with great temperature difference between the paper web and the surroundings there occurs a high dissipation of heat associated with a rapid temperature drop of the paper web beneath the glass transition temperature.
If these prerequisites which are favorable for cooling of the paper web after calendering are not present or have not been attained for technical reasons or can not be attained, there is again lost the quality of the surface obtained, if necessary, in the press nip of the calender or both of the surfaces of the paper web, respectively, due to re-swelling, and here, in particular, due to local different re-swelling, and thus also due to an increase in the macro-roughness, but also due to an increase of the micro-roughness.